Microwave electronic devices, sometimes referred to as radio frequency (RF) devices or vacuum electronic devices, are used in systems with important functions such as radar and high speed communications systems, etc. A traveling wave tube (TWT) may be used as an amplifier that increases the gain, power or some other characteristic of an RF signal, that is, of electromagnetic waves typically within a range of around 0.3 GHz to above 300 GHz. An RF signal to be amplified is passed through the device, where it interacts with and is amplified by an electron beam. The TWT is a vacuum device through which the electron beam travels, typically focused by a magnetic field to prevent the electron beam from directly touching the structure of the TWT.
The electron beam may be generated at the cathode of an electron gun, which is typically heated, for example to about 1000 degrees Celsius. Electrons are emitted from the heated cathode by thermionic emission and are drawn through the TWT to a collector by a high voltage bias, focused by the magnetic field.
The TWT also contains a slow wave structure (SWS) that provides reactive loading in the TWT to slow the phase velocity of the RF signal. For example, a tunnel ladder is one type of slow wave structure in which a pair of wire ladders form a tunnel for the electron beam, with the ladder rungs supported by ridges outside the tunnel. As the RF signal passes through the TWT cavity around the slow wave structure, the capacitance of the SWS slows the phase velocity of the RF signal to about the velocity of the electron beam. Currents are induced in the ladder by the RF signal, creating electromagnetic fields that cause the electrons in the electron beam to bunch up in waves. The velocity-modulated electron beam creates an electromagnetic field that transfers energy from the beam to the RF signal and amplifies the RF signal.
The power of the TWT is limited by the interaction of the electron beam with the RF signal and by the thermal characteristics of the TWT.